Method for recovery of zinc by countercurrent leaching

ABSTRACT

The invention relates to method for leaching zinc-containing materials in connection with the electrolytic recovery of zinc. According to this method, the feed materials i.e. zinc calcine and zinc sulphide, are leached in three stages, in which he sulphuric acid content of the stages rises in accordance with the direction in which the solids are moving. The solids and solution formed in the leaching stages are routed throughout the process countercurrently in relation to each other.

This application is the national phase application under 35 U.S.C. §371of International Application No. PCT/FI2004/000085 filed on Feb. 24,2004, entitled, “Method For Recovery Of Zinc By Countercurrent Leaching”which claims the benefit of Finnish Patent Application No. 20030285filed on Feb. 26, 2003.

The invention relates to a method for leaching zinc-containing materialsin connection with the electrolytic recovery of zinc. According to themethod, zinc calcine is first leached in a neutral leaching stage, andthe resulting solids and zinc concentrate are then leached in aconcentrate leaching and jarosite precipitation stage. The concentrateleaching occurs at a low acid concentration while iron is precipitatedas jarosite. The concentrate leaching is continued in the conversionstage, which takes place at a high acid concentration, so that theferrites also dissolve and jarosite precipitation continues. In thismethod, the solids and solution are fed into the various stagescountercurrently with regard to each other, so that the need forneutralization in the different stages is reduced. The zinc sulphatesolution formed in the neutral leaching stage is directed toelectrolytic precipitation of zinc and iron is separated from the finalleaching stage as jarosite.

Zinc calcine, obtained by roasting sulphidic zinc concentrates, isgenerally used as the starting material in the electrolytic preparationof zinc. The chief component of the calcine is zinc oxide, ZnO, but someof the zinc is also bound to iron in the form of zinc ferrite ZnO.Fe₂O₃.The amount of zinc ferrite is usually so considerable that zinc recoveryfrom it is unavoidable. Zinc oxide is easily soluble even at high pHvalues whereas ferrite has to be leached at a higher acid content.Ferrite leaching is often performed in a separate stage, where both zincand iron are obtained in solution. The majority of the iron has to beprecipitated from this solution before the solution can be returned tothe neutral leach and from there to zinc sulphate solution purificationand electrolysis. The above process is described in e.g. U.S. Pat. Nos.3,434,947 and 3,493,365.

In industrial processes zinc oxide leaching, a neutral leach, isgenerally carried out at a pH of 2-5 and ferrite leaching at an acidcontent of between 30-100 g H₂SO₄/l. The solution from ferrite leaching,which contains the dissolved zinc and iron, is very acidic, and is oftenpre-neutralised, before the iron is precipitated from it. Ferriteleaching can also be combined with the iron precipitation stage. Thismethod is known as the conversion process and is described in U.S. Pat.No. 3,959,437.

Zinc concentrate leaching is currently also being combined in evergreater amounts with zinc oxide or calcine leaching. The concentrate isfed either to ferrite leaching or is leached as a separate pressureleach. The main component in concentrate leaching is zinc sulphide, ZnS.In addition, the iron in the concentrate is bound to pyrite FeS₂, andsome of the zinc in the zinc sulphide may be replaced by iron. Thereforezinc processes based on concentrate leaching or those containing aconcentrate leaching stage also require an iron removal stage. Threeiron precipitation processes are in use, where iron is precipitated aseither jarosite, such as Na[Fe₃(SO₄)₂(OH)₆], as goethite FeOOH or ashematite Fe₂O₃. When iron is precipitated as jarosite or goethite, aneutralising agent has to be used in precipitation in order toneutralise the sulphuric acid released in the reactions. Normally theneutralising agent is calcine.

In the traditional jarosite process iron is precipitated at atemperature close to the boiling point of the solution. Free acid isneutralised to a value of 3-5 g/l H₂SO₄ (optimal pH 1.5). The amount ofiron in the zinc sulphate solution is 20-35 g/l. So that the jarositeattains an essentially crystalline form, which has favourable settlingproperties, potassium, sodium or ammonium ions are also fed into thesolution. Goethite precipitation is described for example in U.S. Pat.No. 4,676,828. In this method, the amount of free acid in the zincsulphate solution entering iron precipitation is 4-8 g/l and the amountof ferric iron 1-2 g/l. Most of the iron is in ferrous form. Oxygen andcalcine are fed into the solution so that the iron oxidises and goethiteis precipitated as the pH rises.

When iron is precipitated as hematite, it occurs from a solution wherethe iron is first reduced from trivalent to divalent form. Then the ironis precipitated hydrolitically by oxidation without neutralisation:2FeSO₄+O₂(g)+2H₂O

Fe₂O₃+2H₂SO₄  (1)The precipitation of iron is however performed in an autoclave at atemperature of about 200° C., the partial pressure of oxygen being about18 bar, which have essentially restricted the adoption of the method,even though hematite is in fact the most environment-friendly form ofiron precipitate.

A zinc recovery process is described in U.S. Pat. No. 6,475,450, whereleaching of calcine and concentrate leaching is combined. Zinc calcineis leached normally in the neutral leaching stage, and the resultingsolution is routed to electrolysis via solution purification. Theneutral leach residue, which consists mostly of zinc ferrite, is routedto the next leaching stage, which is also simultaneously the zincconcentrate leaching stage. The concentrate is leached into theelectrolysis return acid and with an aid of trivalent iron of solidsfrom goethite precipitation. The leaching conditions are adjusted sothat the ferrites dissolve. Trivalent iron is thus obtained from thedissolved ferrites and, in addition, trivalent iron precipitate from asubsequent iron oxidation stage is also returned to this stage. Asolution is obtained from the concentrate leaching stage that containsboth zinc and divalent iron in precipitate. The solution obtained fromthe concentrate leaching stage is oxidised to trivalent in the nextstage, the iron oxidation stage, and is precipitated as goethite, butfor this purpose the solution must first be neutralised, and theneutralisation is carried out using zinc calcine. Some of theprecipitate thus formed is circulated back to concentrate leaching andsome is routed to iron precipitation. The solution from the ironoxidation stage is routed to neutral leaching. The next stage shown inthe patent flow diagram is strong acid leaching, where iron is dissolvedagain in reducing conditions (SO₂) and at the same time ferrites of thecalcine fed into the oxidation stage dissolve. According to the patent,iron is precipitated either as hematite, jarosite or goethite. In theiron precipitation stage the iron has to be reoxidised into ferric form.The zinc sulphate solution from the iron precipitation stage is directedto the neutral leaching stage.

In U.S. Pat. No. 6,475,450 described above, the amount of precipitate tobe circulated from the iron oxidation stage is large, because inaccordance with reaction (1) it takes one mole of ferric sulphate todissolve one mole of zinc sulphide. Calcine generally contains between5-15% of ferrites, so that all the other trivalent iron should becirculated to the stage because no oxygen is fed to the concentrateleaching stage for iron oxidation. After the iron oxidation stage theprocess has a strong acid leach, where the iron that has been oxidisedto trivalent is leached again. When iron is precipitated as hematite, itis done by routing the solution in question to an autoclave andoxidising it there. The patent does also mention, however, that iron maybe precipitated as jarosite or goethite. As stated above, jarosite andgoethite precipitation cannot be performed directly after the strongacid leach but the solution has to be neutralised first, so the processrequires yet another extra stage. If neutralisation is performed withcalcine, at least some of the zinc in the calcine is lost.Neutralisation of process stages with calcine always adds eitheradditional stages to the process, if all the zinc contained in thecalcine needs to be recovered or otherwise the total zinc yield isweakened.

U.S. Pat. No. 5,858,315 also describes a method whereby zinc concentrateleaching is combined with zinc calcine leaching. First the calcine issubjected to a neutral leach, from which the resulting zinc sulphatesolution is fed via solution purification to the electrolyticprecipitation of zinc. The undissolved residue remaining in the neutralleach is routed to ferrite leaching, which occurs in the presence ofreturn acid and oxygen. Concentrate leaching can be performed eitherwith ferrite leaching in the same stage or as a separate sage. Intwo-stage leaching the solution from ferrite leaching, which nowcontains the iron from the calcine in mainly divalent form and the zincsulphate formed in ferrite leaching, is routed to the concentrateleaching stage. Oxygen is also fed into the concentrate leaching stage.In the last leaching reactor the solution is neutralised with calcine.Undissolved precipitate is returned to the ferrite leaching stage andthe zinc- and iron-rich solution is conducted to the iron precipitationstage. Pure calcine is also fed to the iron precipitation stage as theneutralising agent and iron is precipitated with oxygen as goethite. Thezinc sulphate solution from the iron precipitation stage is routed tothe neutral leaching stage.

There are several stages in the method described in U.S. Pat. No.5,858,315, because it is attempted to recover the zinc from the calcineused as neutralising agent. In addition, iron precipitation is alwayscarried out as a separate stage.

U.S. Pat. No. 6,340,450 describes a method of direct zinc leaching,whereby zinc calcine leaching is first carried out in a neutral leachingstage. The solution obtained from the neutral leach is conducted to zincelectrolysis via solution purification and the sediment formed to theconversion stage, to which zinc sulphide concentrate is also fed. In theconversion stage the ferrites of the calcine dissolve together with thezinc concentrate at the same time as the iron is precipitated asjarosite. According to one embodiment of the patent (FIG. 2) zincconcentrate is routed to the end of the conversion stage, when theferrite has dissolved and the jarosite has started to precipitate. Thesulphuric acid content of the conversion stage is regulated to theregion of 10-40 g/l. In the jarosite filtration after the conversionstage the separated zinc sulphate solution is directed back to neutralleaching.

In the method described in U.S. Pat. No. 6,340,450, the conversion andconcentrate leaching stage is performed at a relatively high acidconcentration. The resulting solution containing zinc sulphate is routedto neutral leaching, and because there is acid in the solution, it mustbe neutralised in the neutral leach stage. The higher the acid contentof the solution, the higher the amount of iron in the solution also ingeneral. As a result, the iron circulation in the process increases.

In all the methods described above, the solid and the solution flowessentially in the same direction, which causes a large demand forneutralisation and/or a multi-stage process.

Now a new method has been developed for the leaching of zinc-containingfeed materials in connection with the electrolytic recovery of zinc.According to this method, the feed materials i.e. zinc calcine and zincsulphide, are leached in three stages, in which the sulphuric acidcontent of the stages rises in accordance with the direction in whichthe solids are moving. The solids and solution formed in the leachingstages are directed throughout the process countercurrently in relationto each other.

The first leaching stage is the neutral leaching stage, where thecalcine generated in zinc concentrate roasting is leached, and the zincsulphate solution, which is formed is fed to the electrolyticprecipitation of zinc via solution purification. The calcine is leachedinto sulphuric acid-containing return acid from electrolysis and thesolution from the next leaching stage in the process, which solutioncontains zinc sulphates and iron sulphates. Oxygen and/or air is fedinto the leaching stage in order to oxidise the ferrous iron andprecipitate it as ferric hydroxide.

All the zinc sulphide concentrate to be fed into the process andundissolved solids from neutral leaching are fed to the followingleaching stage, which may be termed a combined concentrate leaching andjarosite precipitation stage, which is performed at a low acidconcentration. The solution for the leaching stage is an acidic solutioncontaining zinc sulphate and iron sulphate from the conversion stage,the next stage in the process. Some of the concentrate is leached in theconcentrate leaching stage, but at the same time the conditions underwhich this occurs are such that iron is precipitated as jarosite.

The solids remaining from the second leaching stage are leached in thefinal stage of the process, the conversion stage, using electrolysisreturn acid and oxygen at a high acid content, whereby the ferrites ofthe calcine and the undissolved zinc compounds of the concentratedissolve and iron precipitates as jarosite. The undissolved precipitateof the final leaching stage contains iron in the form of jarosite andsulphur from the concentrate. The zinc sulphate-containing solutionobtained from this stage is directed to the first concentrate leachingstage and further on to the neutral leaching stage. All the leachingstages are carried out in atmospheric conditions and the temperature iskept between 80° C. and the boiling point of the solution.

The essential features of the invention will be made apparent in theattached claims.

The invention is also described by means of the attached flowsheet 1.

As stated above, the first hydrometallurgical stage in the electrolyticrecovery of zinc is neutral leaching NL, where calcine is leached bymeans of zinc sulphate solution recirculated from the following stage inthe process and sulphuric acid-containing return acid from electrolysis.The movement of the zinc sulphate-containing solution is shown on theflowsheet with a broken line. The neutral leaching stage is carried outin several reactors and to complete the stage the solution and solidsare separated in a thickener. The solids to be fed from one stage toanother in this case refer to the underflow of the thickener, whichincludes both solids fed to the stage but remaining undissolved and alsosolids precipitated as a result of the reactions. According to themethod now developed, the process solids and solution flowcountercurrently in relation to each other. Thus zinc sulphate solutionis fed into the neutral leaching stage from the subsequent stage of theprocess, i.e. the concentrate leaching and jarosite precipitation stage,in which the acid content is kept relatively low, at about 2-20 g/l,preferably between 5-15 g/l. Therefore the acid content of the zincsulphate solution fed into the neutral leaching stage is also relativelylow. This is advantageous for the process, since it means that calcineis not required in the neutral leaching stage to neutralise an excess ofacid in the solution.

One purpose of the neutral leaching stage is to prepare zinc sulphatesolution containing the minimum amount of iron possible, preferably lessthan 10 mg/l. The zinc sulphate solution entering neutral leaching fromlater stages of the process always contains some amount of iron, andwhen the zinc-containing solution comes from a stage where the acidcontent is relatively low, the amount of iron is also lower. It isbeneficial for the process if as much as possible of the iron enteringneutral leaching is in divalent form i.e. ferrous sulphate. Iron ispreferably oxidised in the neutral leaching stage with oxygen and/or airinto iron hydroxide Fe(OH)₃ and precipitated out of the solution. Wheniron is precipitated as ferric hydroxide, minerals harmful in zincelectrolysis such as germanium and antimony can also be co-precipitatedout of the solution. When precipitation of these metals occurs duringneutral leaching, a separate purification stage is avoided in zincsulphate solution purification.

The zinc sulphate solution generated in neutral leaching is conducted tothe various stages of solution purification and the solids separatedfrom the solution are routed to the next stage, which in this method isthe concentrate leaching and jarosite precipitation stage. This stagetoo is carried out in several reactors. The first part of the stage isthe elutriation of the concentrate into the solution that comes from thesubsequent process stage i.e. the conversion stage. The solution fromthe conversion stage is an acidic iron-containing zinc sulphatesolution. The acid level of the concentrate is kept relatively low inthe leaching stage, at around 5-15 g/l, and therefore the acid of thesolution from the conversion stage is neutralised with the sediment fromneutral leaching and concentrate. Since neutral leaching is carried outin this method as a single stage, some of the zinc oxide of the calcineremains undissolved and acts as a neutraliser of the acid in theconcentrate leaching stage, and at the same time the zinc of the zincoxide dissolves into zinc sulphate. Return acid is used to regulate theacid level. Oxygen and/or air are also fed into this stage so that theiron remains in ferric form. In order that the iron in the stage isprecipitated as jarosite, alkali or ammonium compounds are fed to thestage to precipitate jarosite as a crystal, e.g. ammonium jarosite. Thejarosite nuclei required for jarosite precipitation are obtained bymeans of internal circulation of the stage. The precipitating ironoriginates from the neutral leaching solids and the solution from theconversion stage.

At the end of the concentrate leaching and jarosite precipitation stagethere is again separation of solution and solids. A solution is obtainedas the thickener overflow, in which the zinc has dissolved during thestage as zinc sulphate and which also contains a small amount ofdissolved iron, and this solution is routed to neutral leaching. Thesolids obtained as the thickener underflow contains the jarosite thathas precipitated during the stage, the ferrite of the calcine and somestill undissolved concentrate, and these solids are routed to theconversion stage.

The conversion stage leaching occurs by means of oxygen and electrolysisreturn acid in an acid concentration in the region of 25-70 g/l,preferably 30-50 g/l. Fresh sulphuric acid is also used to regulate theacid content, which compensates for the sulphate losses of the wholeprocess. This stage also occurs in several reactors. The ferritescontained in the calcine and the concentrate dissolve in the conversionstage conditions, as does the part of the concentrate that did notdissolve in the previous stage. The jarosite formed in the previousstage no longer dissolves, but the dissolved ferrite is precipitated asjarosite. Jarosite forms at this higher acid concentration, too, sincethere are plenty of jarosite nuclei in the solids, which aidsprecipitation.

Concentrate leaching occurs both in the actual concentrate leachingstage and the conversion stage according to the following reactions:ZnS+Fe₂(SO₄)₃→ZnSO₄+2FeSO₄+S^(o)  (2)2FeSO₄+0.5O₂+H₂SO₄→Fe₂(SO₄)₃+H₂O  (3)i.e. as a sum reaction:ZnS+0.5O₂+H₂SO₄→ZnSO₄+S^(o)+H₂O  (4)

As the reactions show, the oxidation of the concentrate sulphide occursusing trivalent iron, and the divalent iron in the solution is oxidisedto trivalent again with the oxygen fed into the stage. In both stagesmost of the iron is precipitated as jarosite, because only a smallamount of the iron in the calcine and the concentrate is required forconcentrate leaching. Iron precipitation as jarosite occurs according tothe following reaction, where A may be either an alkali or ammonium ion:3Fe₂(SO₄)₃+A₂SO₄+12H₂O→2A[Fe₃(SO₄)₂(OH)₆]+6H₂SO₄  (5)

The acid formed in the jarosite precipitation reaction is consumed inconcentrate leaching.

The slurry formed in the conversion stage may be routed to flotationafter said stage. Flotation is not obligatory, if the jarosite andsulphur concentrate can be stored together. In flotation, the slurry isseparated by flotating the sulphur and the undissolved sulphides. Themajority of sulphides are pyrite. The sulphur concentrate is separatedand the end slurry is fed to jarosite separation. The overflow solutionfrom jarosite thickening is a solution containing acid, iron and zinc,which is recirculated to the concentrate leaching and jarositeprecipitation stage.

The advantage of the countercurrent leaching of concentrate describedabove is the simplification of the process. When the concentrateleaching/jarosite precipitation stage takes place after the neutralleaching stage and as the last stage of the conversion stage, the acidcontent of the stages increases in the direction of flow of the solids.Correspondingly, when the solution is fed countercurrently in relationto the flow of the solids, the acid content of the stages decreases bydegrees. This results in a decrease in the need for neutralisation inthe different stages. Earlier for instance, the acidic zinc sulphatesolution from concentrate leaching was fed into the neutral leachingstage and this acidic solution caused a great demand for neutralisation.In the present method, the solution fed to the neutral leaching stagecomes from a stage where the acid content is kept low. When the acidcontent of the solution fed to neutral leaching is low, its iron contentand in particular the divalent and trivalent iron ratio in the solutioncan also be regulated. As mentioned above, germanium and antimony canalso be co-precipitated in connection with the oxidation of divalentiron that occurs in the neutral leaching stage.

1. A method for recovering zinc from a zinc calcine and zinc suiphideconcentrate in connection with an electrolytic precipitation of zinc inthree stages in atmospheric conditions and at a temperature between 80°C. and the boiling point of the solution, whereby solids and solutionmove countercurrently in relation to each other and acid content of theleaching stages rises in the direction of the flow of the solids, themethod comprising: a) feeding zinc calcine to a first neutral leachingstage, wherein the leaching in the first stage is carried out with asolution conducted from a second leaching stage, and wherein a zincsulphate solution obtained from the first leaching stage is directed viasolution purification to zinc electrolysis; b) feeding both zincconcentrate and solids obtained from the first leaching stage to thesecond concentrate leaching and jarosite precipitation stage, whereiniron contained in raw materials is precipitated as jarosite, and theconcentrate leaching is carried out with a solution conducted from athird leaching stage, and wherein the acid content in the second stageis kept at about 2-20 g/L H₂SO₄; c) feeding solids obtained from thesecond stage to a third leaching stage, wherein the solids obtained fromthe second stage comprise undissolved ferrites, a portion of theconcentrate, and the precipitated jarosite, wherein the ferrites and theportion of the concentrate from the second stage are leached withelectrolysis return acid, wherein the acid content in the third stage iskept at about 25-70 g/L H₂SO₄, and wherein the undissolved precipitatecomprises jarosite.
 2. The method according to claim 1, wherein theleaching in the first stage is carried out in a pH range between 2-5. 3.The method according to claim 1, wherein the acid content in the secondleaching stage is kept at about 5-15 g/L H₂SO₄.
 4. The method accordingto claim 1, wherein the acid content in the third stage is kept at about30-50 g/L H₂SO₄.
 5. The method according to claim 1, wherein theleaching in the first stage is carried out using both the solutionconducted from the second leaching stage and return acid fromelectrolysis, wherein the solution conducted from the second leachingcomprises zinc sulphates and iron sulphates.
 6. The method according toclaim 5, wherein the oxygen and/or air is fed into the first leachingstage in order to oxidise the ferrous iron and precipitate it ashydroxide Fe(OH)₃, which co-precipitates the harmful minerals in thesolution.
 7. The method according to claim 6, wherein the harmfulminerals are germanium and antimony.
 8. The method according to claim 1,wherein the leaching in the second stage is carried out using bothoxygen and/or air and the solution conducted from the third leachingstage, wherein the solution conducted from the third leaching stagecomprises zinc and iron sulphates.
 9. The method according to claim 1,wherein the acid content of the second stage is adjusted usingelectrolysis return acid.
 10. The method according to claim 1, whereinalkali or ammonium ions are fed into the second stage in order toprecipitate the jarosite as alkali or ammonium jarosite, and whereinjarosite nuclei are recirculated within the stage.
 11. The methodaccording to claim 1, wherein the ferrites and the portion of theconcentrate from the second stage are leached in the third stage usingboth oxygen and the electrolysis return acid.
 12. The method accordingto claim 1, wherein flotation is performed on the solids formed duringthe third stage in order to form sulphur concentrate.